A NAND flash memory that is one kind of nonvolatile semiconductor memory device has a stack structure formed by stacking a semiconductor substrate, a tunnel insulating film, a charge storage layer, a block insulating film, and a control gate electrode. In such a NAND flash memory, the applied voltage of the control gate electrode is controlled. By doing so, charges are stored or erased in the charge storage layer via the tunnel insulating film from the semiconductor substrate, and the threshold voltage of each memory cell is changed. In this manner, the NAND flash memory functions as a memory device.
A flash memory needs to be scaled down, and the film thicknesses of all the layers in the stack structure need to be made smaller, so as to improve or maintain performance. In a flash memory, insulating films such as a tunnel insulating film and a block insulating film account for a large proportion of the overall layer thickness. Therefore, the thicknesses of the insulating films such as a tunnel insulating film and a block insulating film are expected to be reduced.
However, an insulating film made of an inorganic material such as an oxide film or a nitride film needs to maintain a certain film thickness, so as to reduce charge releases due to a defect or the like in the insulating film. Therefore, it is difficult to reduce film thicknesses.